Williams, Potemra, Krimigis -
The Twenty-Two Most Frequently Cited APL Publications-III
ionospheric plasma convection patterns, and electric
fields. By 1983, when TRIAD was 11 years old, over
50 scientific articles had been published on studies of
the data, written by 37 different authors throughout the
world, including the U.S., U.S.S.R., the People's
Republic of China, Japan, and Europe. Many of these
scientists participated in a special American Geophysical Union Chapman Conference on "Magnetospheric
Currents" held in April 1983. TRIAD's birthday was
celebrated at that conference and a selection of the
papers was published. 15
Field-aligned currents (now often referred to as Birkeland currents) are important because they provide a link
between the lower auroral ionosphere and the magnetosphere and interplanetary medium. They are also the
source of a variety of interesting plasma phenomena in
the earth's neighborhood. The important role that fieldaligned Birkeland currents have in the flow of energy
between the sun and the earth (as suggested by Gauss
and Birkeland, but refuted by Kelvin and Chapman) becomes more evident with the improvement of satellite
experiments and the advent of multisatellite observational
programs.
REFERENCES
IT. A. Potemra, " MagnelOspheric Current s," Johns Hopkins APL Tech .
Dig. 4,276-284 (1983).
2K. Birkeland, The Norwegian Aurora Polaris Expedition 1902-3, Vol. I,
On the Cause of Magnetic Storms and the Origin of Terrestrial Magnet isms, H . Aschehoug and Co., Christiania (Oslo), orway (1908).
3S . Chapman, "On Cerrain Average Characteristics of World Wide Magnetic Disturbance ," Proc. R. Soc. A 115,242-267 (1927).
4 A . Dessler, "The Evolution of Arguments Regarding the Existence of
Field-Aligned Currenrs," in Magnetospheric Currents, T. A. Potemra, ed .,
Geophys. Monogr. Am. Geophys. Union 28, pp . 22-28 (1984).
5E. H. Vestine and S. Chapman, " The Electric Current-System of Geomagnetic Disturbances," Terr. Magn . Atmos. Electr. 43,351-382 (1938).
6H. Alfven, "A Theory of Magnetic Storms and of the Aurorae," Proc.
R . Swedish Acad. Sci. 18, No.3 (1939); also Eos 51, 180-194 (1970) with
commenrs by A. J. Dessler and J. M. Wilcox.
7S. Chapman, Ann. de Geophys. 24,497 (1968).
8A . J . Zmuda, "Dispersion of Velocity and Anomalous Absorption of
Ultrasonics in Nitrogen," J . Acoust. Soc. Ameri. 23, 472-477 (1951).
9 A. J. Zmuda and L. N . McClung, "Verrical Extrapolation of Geometric
Field Components," Trans. Am. Geophys. Union 36, 939-942 (1955).
IDE. H . Vestine, The Rocket Technique Applied to Exploration of the Geomagnetic Field to Great Heights within the Atmosphere, JHU / APL
CM-480 (1948).
II A . J . Zmuda, J . H. Marrin, and F. T. Heuring, "Transverse Magnetic
Di sturbances at 1100 km in the Auroral Region," J. Geophys. Res. 71,
5033-5045 (1966).
12 W. D. Cummings and A. J. Dessler, "Field-Aligned Currenrs in the MagnelOsphere," J. Geophys. Res. 72, 1007-1013 (1967) .
l3c. O. Bostrom, D . S. Beall, and J. C. Armstrong, "Time History of the
Inner Radiation Zone, OClOber 1963 10 December 1968," 1. Geophys. Res.
75, 1246-1256 (1970) .
14 J. Dassoulas, "The TRIAD Spacecraft," APL Tech . Dig. 12,2-13 (1973) .
1ST. A. Potemra , ed., Magnetospheric Currents, Geophys. Monogr. Am .
Geophys. Union 28 (1984).
ACKNOWLEDGMENTS- These studies would not have been possible
without the vision and dedication of Alfred 1. Zmuda and James C. Armstrong, the cooperation and supporr of Richard B. Kershner, the Space
Deparrmenr, APl, the Navy Navigation Satellite Program, and the continued
supporr of the National Science Foundation and the Office of Naval Research.
The dedication and assistance of the members of the Space Departmenr, whose
outstanding efforts contributed 10 the success of the 5E-I (1963-38C) and
TRIAD programs , are gratefully acknowledged.
HOT PLASMA AND UNUSUAL COMPOSITION
IN JUPITER'S MAGNETOSPHERE
ST AMATIOS M. KRIMIGIS
S. M. Krimigis, T. P. Armstrong, W. I. Axford, C. O. Bostrom,
C. Y. Fan, G. Gloeckler, L. J. Lanzerotti, E. P. Keath, R. D. Zwickl,
J. F. Carbary, and D. C. Hamilton, "Low Energy Charged Particle Environment at Jupiter: A First Look," Science 204, 998-1003
( 1979).
S. M. Krimigis, T. P. Armstrong, W. I. Axford, C. O. Bostrom,
C. Y. Fan, G. Gloeckler, L. J. Lanzerotti, E. P. Keath, R. D. Zwickl,
J. F. Carbary, and D. C. Hamilton, "Hot Plasma Environment at
Jupiter-Voyager 2 Results," Science 206, 977-984 (1979).
The genesis of the two papers by Krimigis et al., published in 1979, that detailed the Voyager observations
at Jupiter actually came eight years earlier when several of the co-authors joined a team to propose participation in the then recently announced opportunity for
an "Outer Planets Grand Tour" program that envisioned sending two spacecraft to successively encoun-
402
ter Jupiter, Saturn, and Pluto and Jupiter, Uranus, and
Neptune. Traditionally, the opportunities for such prestige missions in particle measurements had been
preempted by the "Big Three" of space science at that
time, namely, James Van Allen of the University of
Iowa, John Simpson of the University of Chicago, and
Frank McDonald, then at NASA's Goddard Space
Flight Center. Our proposal represented an attempt by
their former students and our contemporaries to introduce a new generation of state-of-the-art instrumentation into the study of magnetospheres.
Following evaluation of the proposals, our team was
selected to participate in the definition phase of the Outer Planets mission with Robbie Vogt of Cal Tech (who
proposed with McDonald) as the team leader and, in
a gesture by NASA toward the younger generation, myJohn s Hopkin s APL Technical Digesl, Volum e 7, Number 4 (1986)
Williams, Potemra, Krimigis -
The Twenty -Two Most Frequently Cited APL Publications- III
Hot Plasma Environment at Jupiter: Voyager 2 Results
Abstract. Measurements of th e hot (electron and ion energies ;e, 20 and ;e, 28 kiloelectron volts. respectil'e(v) plasma elll'ironment at Jupiter by th e IOIl'-energy
charged particle (LECP) instrument on Voyager 2 hm'e revealed several nell' and
unusual aspects of th e J(}\'ian magnetosphere . Th e magnetosphere is populated from
its outer edge into a distance of at least - 30 Jupit er radii (R J ) by a hot (3 x 10 8 to
5 X 10 8 K ) multicompon ent plasma consisting primarily of hydrogen, oxygen, and
sulfur ions . Outside - 30 R J th e hot plasma exhibits ion densities from -- 10 - 1 to
- 10 - 6 per cubic centim eter and energy densities from - 10 - 8 to 10 - 13 erg per cubic
centimeter. suggesting a high {3 plasma throughout the region . Th e plasma isjioll'ing
in the corotation direction to the edgn)f th e magnetosphere on the dayside , lI'here it
is confin ed by solar Il'ind pressure . and to {/ distance of - 140 to 160 R J on the
nightside at - 0300 local time. Beyond - 150 RJ th e hot plasmajioll' changes into a
.'magnetospheric lI'ind" blowing away f rom Jupiter at an angle of - 20° lI'est of the
sun-Jupiter lin e. characterized by a tempera ture of - 3 X 10 8 K (26 kilo electron
volts), I'elocities ranging from - 300 to > 1000 kilometers per second, and composition similar to that obsen'ed in the inner magnetosphere. Th e radial profiles of the
ratios of oxygen to helium and sulfur to helium (.;: I million electron volts per nucleon) 1J10notonically in crease tOIl'ard periapsis . lI'hile the carbon to helium ratio stays
relatively constan t; a significan t amount of sodium (Na IO - 0.0.5). has also been
identified. Th e hvdrogen to heliu m ratio ranges from - 20 just outside the magnetosphere to I'alues up to - 300 inside; the modulation of this ratio suggests a discontinuity in th e particle population at - 50 to 60 RJ . Large jiuctuations in energetic
particle intensities It'ere obsen'ed on the inbound trajectory as the spacecraft approached Ganymede . some of lI'hich suggest the presence of a " wake ." Five- and
IO-hour periodicities were obsen'ed in th e magnetosphere. Calculations of plasma
jiow velocities with the use ofColl1pton-Gettingformalism imply that plasma is mostly corotating to large radial distances from the planet. Thu s the Jovian magnetosphere is confined by a pla sma boundary (as lI'as implied by .the model of Brice and
l oannidis) rather than a conventional magnetopause . In side the plasma boundary
th ere exists a discontinuity at - 50 to 60 RJ ; we have nam ed the region inside this
discontinuity the " inner plasmasphere ...
We report here preliminary re sults
from me asurement s made with the lowenergy charged particle (LECP) instrument on Voyager 2 as it approached
and traversed the magnetosphere of Jupiter. The primary objectives of the
LECP instrument (I) are to make measurements of the hot plasma (~ 20 ke V
and ;e, 28 keV for electrons and ions , respectively), to characterize the composition of the hot plasma and energetic particle population , and to determine the
particle flow s and spatial distributions .
In addition , we discuss the effects associated with the po ssible wake of Ganymede.
The LECP instrument consists of two
basic sensors. The low-energy particle
telescope (LEPT) is primarily a composition instrument capable of identifying the
major ion species; the low-energy magnetosphere particle analyzer (LEMPA)
performs basic hot plasma (ion-electron)
measurements at low and medium
energies with good electron-ion se paration over a large (- I to 1011 cm- 2 sec- I
sr- I) dynamic range . To obtain a measure of particle anisotropies on a nonspinning spacec raft , both the sensors are
mounted on a stepping motor that rotates
in eight steps through 360° in time inter-
vals of 48 , 192 , or 384 seconds. The
LECP instrument was described in (2).
Inbound pass . The LECP instrument
first observed evidence of Jupiter's magnetosphere when sunward-moving ions
(E ;e, 28 keV) were observed at - 800 Jupiter radii (R J ) in front of the planet. This
distance , more than one-third of an astronomical unit (AU), is substantially
farther sunward than the first ion fluxes
detected by LECP on Voyager I (- 600
R J sunward). As on Voyager I, the frequency of occurrence of the appearance
of such ions increased as Voyager 2 approached the planet.
Figure la shows selected electron and
ion channel count rates for the inbound
traversal of the magnetosphere , which
began on day 184 with the first encounter
of the planet 's bow shock at - 98 R J ;
these bow shocks are identified primarily
by the change in particle flow direction.
Subsequent bow shock crossings are
noted , as are the Jo vian plas ma boundary (rather than magnetopause) crossings , which we will explain later. Also
shown are the a particle to proton (p)
flux ratios and the exponent y of the electron and ion energy spectrum expressed
as a power law in energy (E -Y ). A very
brief excursion into the magnetosphere
SCIENCE , VOL. 206. 23 NOVEMBER 1979
Johns Hopkins A PL Tech nical Digest, Volume 7, Nu m ber 4 (1986)
occurred at - 71 R J ; final entry into the
magnetosphere occurred at a distance of
- 63 RJ on day 186 (identified from the
low-energy electron fluxes).
The pIa ratio exhibits variations over a
factor of 40 with the first two maxima
coinciding with the two plasma boundary
crossings. However, subsequent peaks
in the ratio generally correspond to relative minima in particle intensities , that
is, there appear to be more protons relative to helium off the equator. Generally
the p Ia ratio is much larger inside the
magnetosphere than either the solar
wind value (- 20 to 50) or the Jovian atmosphere value of - 9 (3). The electron
spectra became softer while the ion
spectra became harder during the two
plasma boundary crossings .
Prior to closest approach , the LECP
experiment was commanded into a fixed ,
nonstep mode wherein the low-energy
ends of the LEMPA and LEPT telescopes were oriented to be almost entirely covered by the sunshade . This reduced the geometrical factors of these
telescopes by up to - 95 percent and
provided the opportunity to continue
composition measurements by LEPT
through spacecraft periapsis . Selected
ion and electron data obtained through
periapsis are plotted in Fig. lb. Unlike
the Voyager I inbound observations ,
evidence of an approximate 5-hour periodicity began to appear in the particle
fluxes beginning at - 33 R J ; the periodicities persisted until - 16 R J • Jovian particle flux periodicities were previously
observed by instruments on the inbound
Pioneer IO spacecraft (4 , 5). After the
last days ide plasma boundary crossing
(Fig. la) and prior to the onset of the periodicities , the fluxes , although variable,
did not increase significantly with decreasing distance to the planet. At about
the time of onset of the 5-hour periodicities , however, the fluxes began to increase toward their peak values reached
near periapsis (Fig. Ib).
Ganym ede encounter. Large fluctuations , some periodic , in the electron and
ion intensities began at - 0400 on day
190 and terminated at - 1200 (all times
are in SCET, spacecraft event time). The
closest approach to Ganymede occurred
at - 0714 SCET. Passage through the
nominal (0 4 model) particle drift shell
corresponding to the Ganymede orbit began at - 0741 and terminated at - 0821.
The spacecraft trajectory was expected
to cross the Ganymede wake region - I
hour after closest approach . In Fig. 2b
are shown 24-second average counting
rates of several electron , proton, and ion
channels at a pitch angle of - 90° during
977
403
Williams, Potemra, Krimigis -
The Twenty- Two Most Frequently Cited APL Publications-III
self as the deputy team leader. Members of the Definition Group included John Simpson and Peter Meyer
from the University of Chicago, James Van Allen from
the University of Iowa, and Ian Axford, then at the
University of California at San Diego. Needless to say,
in defining the instrument complement, each member
of the definition team was constantly on alert to make
sure that their own interests and aspirations were represented in the final document. Just before the completion of the Mission Definition report, however, the
Nixon Administration decided that it could not afford
the price tag on the Outer Planets mission, and the program, through some last-minute maneuvering, was
reduced to the so-called Mariner-Jupiter-Saturn mission, which was to consist of two much less capable
spacecraft that would each have only a four-year lifetime and encounter just Jupiter and Saturn.
Each of the teams represented on the Definition
Group wrote new proposals, and our team was selected; neither Iowa nor Chicago made the final selection.
We were notified in December 1972 that the first meeting was to take place that month at JPL to begin the
process of planning the exciting mission. It should be
noted that even though our proposal had been judged
by the peer review selection committee as outstanding,
there was quite a bit of maneuvering within NASA on
how to proceed because of the difficulty in turning
down the proposals of the Chicago and Iowa groups.
Instrumental in arguing for participation by our team
in the mission was the NASA program scientist, Mike
Mitz (deceased), and the deputy director of planetary
programs, Ichthiaque Rasool (now in private business
in Paris). Our team consisted of Carl Bostrom and myself of APL, T. P. Armstrong of the University of Kansas (a close associate of mine from our Iowa days),
George Gloeckler of the University of Maryland (a former student of Simpson and a close associate of his),
C. Y. Fan, then at the University of Arizona, and W. I.
Axford who served as the theorist on our team; L. J.
Lanzerotti of Bell Laboratories joined the team in the
second phase of the proposal for the Mariner-JupiterSaturn program.
The proposed instrument I included sensors with an
energy threshold down to the 20 kiloelectronvolt range,
building on the experience of the Energetic Particle Detector of Williams and Bostrom (built at APL) and the
Charged Particle Measurements Experiment built by
Krimigis and Armstrong (also at APL) that had just
been launched on the IMP-7 spacecraft. Another sensor
complement proposed to measure individual ion species
down to energies of 100 to 200 kiloelectronvolts per
nucleon, using very thin (2 to 5 micron) silicon detectors that had been used earlier by Krimigis and Armstrong in gross composition studies of the Earth's magnetosphere. The capability of the University of Maryland group under Gloeckler in making large rectangular detectors, not available commercially, and their
experience in composition studies contributed to making the composition "telescope" very elegant, somewhat beyond the state of the art for its time.
404
We should recall that nothing was known about the
magnetosphere of Jupiter at the time the experiment
was designed. The Pioneer 10 and 11 spacecraft were
launched in 1972 and 1973, respectively, and were to
encounter Jupiter for the first time in 1973 and 1974.
The Pioneer instrumentation consisted of detectors that
had an energy threshold an order of magnitude higher
( - 500 kiloelectronvolts compared to our - 20 kiloelectronvolts) for ions, and their dynamic range was substantially less than our team's design. It was precisely
these instrumental differences that enabled our measurements in the Jovian magnetospheric system in 1979
to provide fundamental new information regarding the
plasmas and trapped radiation, even though the two
Pioneer spacecraft had already encountered the planet
five years earlier.
A major discovery of the Pioneer missions had been
the unexpectedly large size of Jupiter's magnetosphere,
previously estimated to extend to about 50 Jovian radii
(a Jupiter radius is 71,400 kilometers) upstream of the
planet on the sunward side but actually observed to extend as far as about 140 Jovian radii . The agent for
this tremendous inflation of Jupiter's magnetosphere
was suspected to be some low-energy particle population, but the Pioneer instrumentation was unable to
provide the measurements necessary to answer that essential question. As it turned out, our experiment measured the "hot plasma" component in the magnetosphere of Jupiter that provided the principal source for
the inflation of the magnetosphere to such large distances from the planet. The temperatures of the plasma
ranged from about 20 kiloelectronvolts to as high as
45 kiloelectronvolts (200 x 10 6 to 500 X 10 6 K), and
the density was about 10 - 3 per cubic centimeter; this
was the hottest plasma yet observed in our investigations
of the solar system up to that time. In addition, the
composition telescope obtained measurements showing
that the elemental abundances in the hot plasma contained heavy ions such as oxygen and sulfur in numbers that were comparable to those of protons. The
source of the plasma, as became evident a few days after
closest approach to the planet by Voyager 1, was elemental sulfur and sulfur dioxide injected into the magnetosphere by the volcanoes on Jupiter's satellite 10 (see
the front cover). Thus, an essential aspect of the basic
physics of the magnetosphere of Jupiter was discovered
only because the instrumentation that our team had put
together was able to make measurements in a regime
of energy and composition that was not previously observed. If the Pioneer 10 and 11 instruments had had
that same capability, they would have discovered both
the hot plasma and the unusual composition, which
might have led to an inference of Ionic volcanoes back
in 1973-74 prior to the observations of Voyager.
It is interesting to note that we had several discussions among members of the team on the relevance of
interpreting the low-energy ion data in terms of a Maxwellian hot gas with a high energy, non-Maxwellian tail.
This approach was initially agreed upon in a long conversation I had with George Gloeckler concerning an
John s H op kin ~ APL Tcdlllical Ui!2t'.,t. Vo lum e 7. .\lumber.J (1986)
Williams, Potemra, Krimigis -
analysis that he performed on the angular distributions
of ion measurements in the outer magnetosphere, where
corotation velocities of the plasma are large (600 to 1000
kilometers per second). During the encounter, the team
had already recognized that the angular distribution of
the ions was far too anisotropic to be explained by the
mere heating of protons, and that it must have had a
very strong component of heavier ions, as subsequent
composition measurements made clear. The validity of
our interpretation in terms of a hot plasma model was
clinched when we were able to confirm from Fred Scarf,
principal investigator of the plasma wave instrument,
that the cut-off frequencies of continuum radiation in
the outer magnetosphere were consistent with densities
of about 10 - 3 per cubic centimeter, i.e. , similar to
those deduced from our analysis of the charged particle data. This astounding result implied that there was
no low-energy (less than 20 kiloelectronvolts) plasma
in the outer magnetosphere of Jupiter, and none was
measured by the MIT plasma probe on Voyager. Extensive analysis of the Voyager 2 data firmly established
the inferences from Voyager 1 that the hot plasma in
Jupiter's magnetosphere determines the dynamics of the
interaction between the Jovian magnetic field and the
solar wind.
The second and most important discovery of the
Voyager encounters with the magnetosphere was the observation of plasma outflow from the nightside, which,
although present in the Voyager 1 data , was not clearly identified until the Voyager 2 encounter in July 1979.
It became evident that there was substantial outflow
J ohns H opkins A PL Technica l Digesl , Vo l um e 7. N umber 4 (1986)
Th e T wenty-Two Most Frequently Cited APL Publications-III
of this hot plasma from the nights ide of the magnetosphere and that it most likely constituted the principal
energy loss process at a rate of about 10 20 ergs per second (about 10 13 watts). The characteristics of that
plasma outflow were examined by us in great detail,
and we decided that it presented a new phenomenon
that we labeled the "magnetospheric wind." In the Voyager 2 paper, we provided a conceptual sketch of what
we believed to be the basic plasma physics of Jupiter' s
magnetosphere; the model has remained essentially unchanged to this day, despite considerable discussion
within the scientific community on the details of the
model.
In summary, the popularity of citations for the two
papers is principally due to the novelty of the observations, which would not have been there were it not for
the daring (foolishness?) of the experiments and engineers in pushing the instrument design significantly
beyond what prudent, state-of-the-art instrumentation
concepts would have dictated at that time. Contributing to the frequency of citations is undoubtedly the novelty of the interpretation of the observations and the
significant level of discussion introduced in the theoretical community on the details of the concepts expounded in the two papers.
REFERENCE
IS . M . Krimi gis, T . P. Armstrong, W. I. Axford , C. O . Bostrom, C. Y. Fan,
G. Gloeck ler, and L. J . Lanzerotti, " Th e Low Energ y Charged Particle
(LECP) Ex periment o n the Voyager Spacecraft," Space Sci. Rev. 21 , 329-354
( 1977).
405